1. Field of the Invention
The present invention relates to color evaluation processing of printed matter of an image forming apparatus.
2. Description of the Related Art
In recent years, a demand for direct imaging printers which do not require preparation of any printing plates is growing. Many businesses adopt direct imaging printers so as to shorten the preparation time of printed matter, to improve services to individual customers, and to solve environmental problems, i.e., mass-circulation production and waste. Of direct imaging printers, ink-jet printers which have relatively low prices and are suited to photo printing, and electrophotographic printers which have high productivity and can provide printing results similar to offset printing are in great demand.
In such situations, color reproducibility is extremely important in alternatives to the conventional offset printing and photo printing. Stabilization control is often executed in a printer to assure the color reproducibility. More specifically, a density sensor measures a patch pattern formed on a photosensitive body to detect a toner density. The detected toner density is fed back to a toner density controller in a developer to control the toner density (see Japanese Patent Laid-Open No. 1-309082).
Japanese Patent Laid-Open Nos. 62-296669, 63-185279, and the like propose image control by reading an image using a reader built in a copying machine. Also, Japanese Patent Laid-Open No. 2002-344759 discloses a technique which corresponds to color detection and adjusts gray balance to which the human visual perception is sensitive.
Even ink-jet printers suffer variations in color reproducibility caused by changes in the amount of ink ejection over time, individual differences in ink cartridges, and the like. For this reason, in order to accurately grasp the color reproducibility after color development of inks and to control the amount of ink ejection, products in which a density sensor is attached beside the printhead to measure the image density are commercially available.
Also, color reproducibility is important for illuminant devices such as monitors and the like. A scheme called monitor remote proof that does not use any printed matter is adopted. This scheme has a mechanism for making a client confirm color reproducibility of reference printed matter on a monitor and prompting the client to determine “pass” or “fail” of the color reproducibility. That is, the client can instantaneously execute a process so-called proof of printing on the monitor. Since digital proof data is displayed on the monitor, a shorter delivery period can be realized than paper-based proof.
The color reproduction scheme of a device which makes color reproduction on a print medium or on an illuminant, obtaining constant colors all the time and approximating color reproduction of printed matter are high-priority issues, and the printer manufacturers must guarantee them. In order to guarantee such issues, a standardized color reproducibility evaluation method is indispensable.
However, the conventional evaluation method merely represents good or poor color reproducibility or the degree of color or density heterogeneity it one page (to be referred to as heterogeneity in page image hereinafter; in other words, a color difference in page image). Owing to the characteristics of a printer, it is difficult to attain zero color difference in page image. In the case-of ink-jet printers, unevenness in scanning of a printhead, unevenness in conveying of print sheets, unevenness of the ink ejection characteristics of the printhead, and the like may occur. In the case of electrophotographic printers, it is difficult to make colors and densities in page image constant due to unevenness in scanning of a laser beam, deterioration of respective parts such as the developer, drum, transfer rollers, and the like, unevenness in melting of toner due to the biased temperature of a fixing roller, and the like. Respective manufacturers have made various efforts to avoid heterogeneity in page image, but they have not realized printers with zero color difference in page image. Note that various kinds of unevenness in printing will sometimes be referred to as “unevenness in printing”.
That is, conventionally, a color difference including heterogeneity in page image has been discussed as color reproducibility. However, how much heterogeneity in page image color reproducibility exists, including whether or not the condition of heterogeneity in page image of a printer falls within an permissible range, what is to be fixed to improve color matching precision, and so forth, are unknown. For this reason, the color reproducibility is evaluated with disregard to the heterogeneity in page of the printer, and a multi-dimensional lookup table (LUT) such as an ICC (International Color Consortium) profile or the like is prepared again. Preparing an ICC profile requires processes such as print output, colorimetry, arithmetic operations of an LUT, installation of a profile in a printer controller, and the like, resulting in long processes. Since printing halts during that time interval, re-preparation of an ICC profile should be avoided as much as possible. In other words, the color reproducibility must be evaluated with consideration of the influence of the heterogeneity in page image.
Of course, a device such as a monitor or the like suffers the influence of heterogeneity of illuminant. The following description may often use “homogeneity in page image” as an antonym to “heterogeneity in page image.”
Upon preparation of an ICC profile, whether or not a printer is in a normal state must be detected. If the ICC profile is prepared in a state far removed from the normal state, improvement of the color reproducibility cannot be expected. That is, the color reproducibility changes after every image control disclosed by Japanese Patent Laid-Open No. 62-296669.
Japanese Patent Laid-Open No. 2001-144987 discloses a configuration that evaluates the image quality of a printer to be evaluated based on color information of a reference evaluation pattern. According to Japanese Patent Laid-Open No. 2001-144987, if toner or the like flies in all directions, a density difference or color difference causes a mismatch to occur. Hence, that mismatch is corrected to solve the problem of color matching with subjective evaluation, thus evaluating the image quality.
Japanese Patent Laid-Open No. 2003-216398 discloses a technique which converts reference color information into a barcode, appends the barcode to print data, and prints the barcode upon printing a color evaluation pattern. That is, this reference discloses a configuration that prevents setting errors of a target and the like. In this disclosure, the barcode information is compared with measurement information of the pattern to determine whether or not colors that the creator of print data intended are reproduced.
Japanese Patent Laid-Open No. 8-219886 proposes a heterogeneity in page image evaluation scheme associated with color reproducibility. More specifically, heterogeneity of white and heterogeneity in page image due to unevenness in printing are measured using a small color difference spectrometer and XY stage.